When the next revolution rocks physics, chances are it will be about nothing—the vacuum, that endless infinite void. In a discipline where the stretching of time and the warping of space are routine working assumptions, the vacuum remains a sort of cosmic koan. And as in the rest of physics, its nature has turned out to be mind-bendingly weird: Empty space is not really empty because nothing contains something, seething with energy and particles that flit into and out of existence. Physicists have known that much for decades, ever since the birth of quantum mechanics. But only in the last 10 years has the vacuum taken center stage as a font of confounding mysteries like the nature of dark energy and matter; only recently has the void turned into a tantalizing beacon for cranks. As one blond celebrity heiress and embodiment of emptiness might say, nothing is hot.
To investigate the mysteries of the void, some physicists are using the biggest scientific instrument ever built—the just-completed Large Hadron Collider, a huge particle accelerator straddling the French-Swiss border. Others are designing tabletop experiments to see if they can plumb the vacuum for ways to power strange new nanotech devices. “The vacuum is one of the places where our knowledge fizzles out and we’re left with all sorts of crazy-sounding ideas,” says John Baez, a mathematical physicist at the University of California at Riverside. Whether in the visionary search for the engine of cosmic expansion or the near-fruitless quest for perpetual free energy, the vacuum is where it’s happening. By mining the vacuum’s riches, a true theory of everything may yet emerge.
Empty space wasn’t always so mystifying. Until the 1920s physicists viewed the vacuum much as the rest of us still do: as a featureless nothingness, a true void. That all changed with the birth of quantum mechanics. According to that theory, the space around a particle is filled with countless “virtual” particles rapidly bursting into and out of existence like an invisible fireworks display.
Those virtual quantum particles are more than a theoretical abstraction. Sixty years ago a Dutch physicist named Hendrik Casimir suggested a simple experiment to show that virtual particles can move objects in the real world. What would happen, he asked, to two metal plates placed very close together in a complete vacuum? In the days before quantum mechanics, physicists would have said that the plates would just sit there. But Casimir realized that the net pressure of all the virtual particles—the stuff of empty space—outside the plates should exert a minuscule force, a nudge from nothing that would push the plates together.
Physicists tried for decades to measure the Casimir force with great precision, but it wasn’t until 1997 that technology caught up with theory. In that year, physicist Steve Lamoreaux, now at Yale, managed to detect the feeble Casimir force on two small surfaces separated by a few thousandths of a millimeter. Its strength was about equal to the force that would be exerted against the palm of one’s hand by the weight of a single red blood cell.
At first most physicists regarded the Casimir force as a quantum oddity, something of no practical value. Now that has changed: Forward thinkers see it as an important energizer for the tiniest of machines, devices on the nano scale, and a few labs are working on ways to use the force to defy the conventional limitations of mechanical design. Federico Capasso, a physicist at Harvard, leads a small team that is trying to create a repulsive Casimir force by tinkering with the shapes of plates or with the coatings used to cover them. His entire set of experiments fits on a desktop, and the objects he works with are so small that most of them cannot be seen without a microscope.
“Once you have a repulsive force between two plates, you should be able to eliminate static friction,” Capasso says. That could lead to a host of useful applications, including tiny frictionless bearings or nanogears that spin without touching. “But the experiments are enormously difficult, so I cannot tell you when and how.”
For all its strangeness, the Casimir force may be the one property of empty space that does not baffle today’s physicists. It is garden-variety quantum mechanics, weird but not unexpected. The same can’t be said about dark energy, a truly astonishing discovery made by astronomers a decade ago while observing distant exploding stars. The explosions revealed a universe expanding at an ever-faster rate, a finding at odds with previous expectations that the expansion of the cosmos should be slowing down, braked by the collective gravitational pull of all the matter out there. Some unknown form of energy—physicists call it dark energy simply for lack of a more descriptive term—appears to be built into the very fabric of space, countering the gravitational pull of matter and pushing everything in the universe apart. Some theorists speculate that dark energy might cause a runaway expansion of the universe, resulting in a so-called Big Rip some 50 billion years from now that would tear the cosmos to pieces, shredding even atoms.
The observations have allowed physicists to estimate the quantity of dark energy by deducing the force needed to produce the accelerating effect. The result is a minuscule amount of energy for every cubic meter of vacuum. Since most of the cosmos consists of empty space, though, that little bit adds up, and the total amount of dark energy completely dominates the dynamics of the universe.
With the discovery of dark energy came difficult questions: What is this energy, and where does it come from? Physicists simply do not know. According to quantum mechanics, the energy of empty space comes from the virtual particles that dwell there. But when physicists use the equations of quantum theory to calculate the amount of that virtual energy, they get a ridiculously huge number—about 120 orders of magnitude too large. That much energy would literally blow the universe apart: Objects a few inches from us would be carried away to astronomical distances; the universe would literally double in size every 10-43second, and it would keep doubling at that rate until all the vacuum energy was gone. This may be the most colossal gap between observation and theory in the history of science. And it means that physicists are missing something fundamental about the way the universe works.
“We’ve made a prediction on the basis of our best theories, and it is wrong, wildly wrong,” says Sean Carroll, a theoretical physicist at the California Institute of Technology. “That means we don’t just tweak a parameter here and there; we really have to think deeply about what our theories are.”
Even if no one knows where the energy of empty space comes from or why it has the value it does, there is now no doubt that it exists. And if there is energy to be had, there is inevitably somebody out there thinking of how to exploit it. The notion of limitless energy from empty space has inspired legions of wannabe physicists who dream of developing the ultimate perpetual-motion device, a machine that would solve the world’s energy problems forever. A quick Internet search for the words free energy and vacuum turns up pages and pages of schemes for tapping the vacuum’s energy. I ask John Baez if such efforts are as hopeless as previous perpetual-motion machines. Are they equally crazy and doomed to failure?
“Perhaps not as doomed as trying to prove the world is flat,” Baez says. “One thing I can say is that I sure hope it doesn’t work, because if you could extract energy from the vacuum, it would mean that the vacuum is not stable. For normal physicists,” he adds with a laugh, “the definition of the vacuum is that it’s the lowest-energy situation possible—it has less energy than anything else.” In short, Baez says, while we may be able to get energy from the vacuum, success “would mean the universe is far more unstable than we ever dreamed.”
The reasoning goes like this: If the vacuum is not at the lowest energy state possible, then at some point in the future, the vacuum could fall to a lower state, pulsing out energy that would threaten the very structure of the cosmos. If some clever engineer were ever to extract energy from the vacuum, it could set off a chain reaction that would spread at the speed of light and destroy the universe. Free energy, yes, but not what the inventors had in mind.
So maybe we won’t be pulling energy from the vacuum, but we might soon get some different benefits from empty space: confirmation of a 40-year-old theory and, with any luck, some radically new physics.
This fall the Large Hadron Collider (LHC) begins slinging protons at 99.99 percent the speed of light in opposite directions along a circular, 17-mile course. In the debris of the collisions that follow, physicists expect to find evidence of yet another strange component of empty space, one that would explain why particles have mass. Besides virtual particles and dark energy, theorists believe that the universe contains something called the Higgs field. Like dark energy, the Higgs field is thought to permeate all of space. But unlike the discovery of dark energy, which was completely unexpected and is still inexplicable, the detection of the Higgs field won’t surprise physicists at all. They have been hunting it ever since Peter Higgs, a physicist at the University of Edinburgh, proposed its existence in 1964.
Higgs wanted to explain why matter has mass, and more specifically why every particle has a different mass. He theorized the existence of an invisible field filling all of space and argued that particles acquire mass by interacting with this field. What we interpret as a particle’s mass is really the strength of its interaction with the Higgs field. For a very loose analogy, think of pushing a marble through syrup: The stickier the syrup, the harder it would be to push it.
If the Higgs field does exist, the LHC should find a previously unseen particle called the Higgs boson. Just as light, which is an electromagnetic field, is transmitted by particles called photons, physicists expect that the mass-endowing effect of the Higgs field is ferried by Higgs bosons.
The discovery of the Higgs boson would answer one of the most basic puzzles of our reality, and yet physicists seem oddly blasé about the prospect. “If it’s found, that would actually not be that exciting,” Baez says. “It would be a relief, maybe. Well, it would be exciting, but only in the same sense as if you lose your keys and then you find them again. Someone would certainly win a Nobel Prize for it, but after the initial excitement, particle physicists would become grumpy because it would just mean that what we thought was true is true, and all the things we don’t understand we still don’t understand, and there is still no new evidence.”
Some researchers, though, expect the LHC to turn up evidence of something very new indeed—extra dimensions of space. According to M theory—the latest, most audacious attempt to explain the fundamental workings of physics—the space around us may be made of as many as 11 dimensions. M theory proposes that the ultimate building blocks of the universe are not particles but tiny vibrating loops of energy, or strings, as physicists call them. For complicated mathematical reasons, those loops need 11 dimensions in which to vibrate; otherwise the theory doesn’t work. We experience only four dimensions (three of space and one of time) in everyday life because the other seven are supposedly so small that we do not notice them. They become evident only on the subatomic scale.
One way to picture this is to imagine a tightrope walker on a high wire. To the tightrope walker the wire is essentially one-dimensional, a line pointing in one direction. But an ant crawling on the wire would see it as a three-dimensional object; the ant could crawl completely around the wire, experiencing a dimension that is inaccessible to the tightrope walker. String theorists would say we’re like the tightrope walker, except that our “rope” is an 11-dimensional space, of which we are able to perceive only four dimensions.
Advocates of M theory have had a tough time convincing some of their colleagues about the reality of all those extra dimensions, but the LHC might win some converts. If extra dimensions really exist, some of the particles produced by the collisions inside the big accelerator may slip away into other dimensions, and particles from higher dimensions could spill into our four-dimensional world. So if physicists notice a shortfall or a surplus in their particle tallies at the accelerator, it might be the first evidence of the wild new physics to come. “Probably whatever is true will in fact be crazy, because historically the truth in physics always seems to be more far-out than anything you could have imagined,” Baez says.
Some physicists like to think that M theory will form the basis of what they call a theory of everything, a set of laws that will completely describe the universe in all its strangeness, where dark energy, quantum theory, extra dimensions, and magazine readers will all fit into one tidy package. But in the end, the key to cosmic truth may well come from another window on reality, the looming void. A good theory of nothing just might be the theory of everything physicists have sought for so long.
